Dental investment composition and process



Patented July 2, 1935 UNITED 1 STATES PATENT oi-"ncr: v I 7 DENTAL msmm. COMPOSITION AND raocass Paul Francis Collins, Columbus, Ohio, asslgnor to The Columbus Dental Manufacturing Oompany, Columbus, Ohio, a oorporation No Drawing. Application June so, 1933, Serial No. 678,547 i 14 Claims. (01'. 22-1 8 .pin or the like. Hereinafter the term precious metal will be understood to include alloys containing one or more precious metals, with or without one or more non-noble metals.

In the process of casting, (which term-is used in a broad sense to include also soldering together of a plurality of precious metal pieces, in making up artificial dentures and the like), it is customary to imbed a preciousmetal piece or a plurality of such metal pieces, in an investment composition, leaving preshaped spaces into 30 which the precious metal is to be run, the investment composition being previously mixed up. with water to form a mass of pasty consistency. The

investment composition sets to a relatively hard mass,vand subsequently is drledand heated, prior to the introduction of the new precious metal in a liquid state, to be cast onto the insets. It is customary to form the cavity into which the precious metal is to be poured, by attaching to the insets, wax which has theform desired in the cast metal. It is customary to heat up the mold thereby formed, sufilciently" to thoroughlydry this out, and to melt out the wax, which wax may partly run out through a spruehole provided, and/or some of the wax may be absorbed 5 by the investment composition, after the latter has set, and during the heating operation. The heating of the mold may be carried to a rather high temperature, since by such procedure the mold is already hot before pouring in the molten o precious metal, which latter is accordingly not chilled until it has completely filled the space previously occupied by the wax.

.The investment composition usually employed contains a. silicious base, for example finely 5 ground quartz, sand and the like. The invest-' ment composition also contains a binder, usually plaster of Paris, or some other gypsum product, namely calcium sulphate, which of course becomes completely hydrated during the steps of 3 mixing with water and air setting. Agents to retard or accelerate the setting or hardening, and/or to modify the plasticity of the wet mixture, may also be used, as is well known in the art. The heating up of the molds may be performed 5 in various ways, and it is not always convenient for the dentist or the workers in the dental laboratory, to carefully control the temperature conditions in the molds, during the heating up and casting processes. The heating may be accomplished-by placing the mold, after removal 5 oi? the sprue pin or sprue pins, over a gas flame,

or on a hot plate, or in an oven or in some other manner and usually it is a fact (although frequently not appreciated by the worker) that the atmosphere within the mold, and adjacentto the 1 insets above referred to, may become reducing, when the material is subjected to a high tem-. perature such as about 900 F. up to 1600 F.,

or higher, this reducing atmosphere being perhaps produced by the residues of the wax, or by 16' the gas coming from the gas burner, when incompletely consumed, or from other causes. There is also liability of a more or less large portion of the calcium sulphate in the investment composition to be decomposed, in the pres- 20 ence of silica, liberating sulphur trioxid which, in the reducing atmosphere sometimes forms free sulphur, or reduced sulphur compounds including calcium sulphide, hydrogen sulphide, carbon bisulphide, sulphur dioxide, and perhaps others, and this may cause deterioration of the piece or pieces, oi precious metal or precious metal alloy initially inserted into the investment composition, and against which it is desired to cast precious metal or precious metal alloy. It is also possible that sulphur compounds if present in the heating gas may produce or aid in producing the said deterioration. v

The deterioration appears to be progressive, and appears to be much greater whentemperatures higher than ordinary are employed, and when continued for a relatively long period. Ordinarily it has been practice in this type or casting, to use temperatures up to about 1500" F. continued for about an hour or more. In accordance with the present invention, I may employ, as the basis of the investment composition, any of the usual-investmentcompositions such as 18 to 50 parts of plaster of Paris or other calcium sulphate binder, and 50 to 82 parts of fine sand or' other inert silicious material. With the said materials I employ a protective agent, as described below, for preventing deterioration of the precious metal and particularly for preventing deterioration of precious metal insets as described above.

The whole of'the investment composition is preferably in a finely divided condition, so that 85% shall pass a number 200 sieve, shall pass a number sieve and 100% shall pass a 55 number 30 sieve. 'Ihis screen analysis is generally accepted as standard, by the profession. Other materials such as asbestos fibers or other agent to impart strength, coloring matters .as well known in the art, can be added if desired.

The protective agents which I have found to be most suitable are substances which are capable of readily combining with sulphur, to produce sulphides.

I have found that by the addition, as protective agents, of the metals copper, nickel, cobalt, iron, molybdenum or silver, (as free metals, 1. e. in the pure metallic state) or alloys of these metals among themselves, or with zinc or alloys of one or more of these metals with other metals, or the oxides of copper, nickel, cobalt, or silver, in

the amount of about 7 to 15% ofthe total iii-Q vestment composition, it is entirely possible to protect metal insets from deterioration, even when heated in the mold for a long period such as 6 to 8 hours. These substances. arethe most eflective, for producing the said effect, of all of the-many substances with. which I have experimented. Smaller amounts of the; above mentioned materials can be added, if the heatingis to becontinued for not more than about one and a half to two hours, as well as 7 to 15% of an oxideof manganese, such asmanganese dioxide, chromium oxide, molybdenum trioxide, molybdenum sesquioxide, molybdenum dioxide, cuprous oxide, or zinc oxide. -If it is necessary to afford protection'to the-precious metal insets for only a' much shorter period, such as 30 minutesto one hour at high temperature, metals such as chromium or manganese, can be employed, also more than 7% of ferric oxide or ferroferric oxide. These materials can be used in the amount of 7 to 15% or more,'or in place of these materials, smaller quantities of the ,materials of the first and-second groups above mentionedcan be employed. About. 3.to 5% of powdered chromium oxideis also to some extent useful in preventing or retarding injury to the metal insets.

- In place of the metals or oxides, I may employ certain other materials-for example carbonates,-,oxalates or other organic acid salts of the metals, which uponheating to the temperatures above'statedwill decompose to leave oxide or metal. I find also that chlorides and other haloid acid salts of the above metals can be used in place of the metals or oxides. However the use of the haloid salts does not appear to be of any particular advantage over the use ofthe metals or oxides, and in certainycases they are not as satisfactory.- I find also that certain alkaline earth metal halides) for example strontium chloride) give fairly good results.

The-metal tungsten or its oxide or nitrate or chloride also-can be used. Nitrates of all of the metals referred to above can be suitably employed, thesejof course giving oxides during the heating operation. .I have found also that difflcultly reducible salts of the acids which contain amphoteric metals in the acid radical can suitably be employed, for example sodium molybdate, sodium tungstate, both of which were found to give fairly good results. The above mentioned salts; as will be noted, dov not containsulphur. 'I believe that sulphur-containing salts would not be, useful for the purposes indicated. 1 Insoluble salts (e. g. carbonates) are usually preferable to soluble salts. Of the first class of substances-above referred to, 1; preferably employ at least 7%,-and up to of materials can be found sufiicient in most cases.

The amount of the protective material to be added can vary between rather wide limits, de-

pending upon the composition of the embedded metal to be protected, the temperatures which 'are'to be employed in-heating up and casting,

and the length of time taken to complete the operation- Quantities: from 0.5% up to 40% of the entire investment composition can be emploved.-. Withmost of the cases I would recommend the use of over 5% and not substantially over 25% of the protective agent, for giving most satisfactory results. With iron oxide '1 would preferably employ from 10 or 12% up to The protective agent may be used in the investment composition as aggregate, as a gran-v ular material or preferably ass-pulverized material. The pulverized material isv preferably;

graded from 100 to ,400'mesh., It is best, th0,r-'

oughly mixed withtheinvestment composition.

for'exampleby grinding or mixing the entire mass together or by'srinding or some of the investment material v(e. 3...: small portion of then adding this, to the remainder of;the invest-' ment composition and grinding or mixing the o the sand or quartz), with the, protective agent;

remaining constituents of investment compo sition. When employing metals in thejel ei'nentary form, or alloys of metals] the protective,

agent will function satisfactorily 'if used in the form of small strips or pieces of foil placed'around and in proximity. to the metal insets, but such,

is not the preferred form. V

In making upthe composition, it will be under stood that asmall'amount of a highly active protective agent produces a result equivalent to a larger amount of a less activev agent;

As analternative mode of procedure, the operator could dissolve the protective agent in the water, or mix the protective agent with the water,

if notsoluble therein, and use this water forv mixing with the investment composition proper," in such proportions as indicated above. I con. sider the use of the protective agent ingvery finely divided condition, thoroughlyfmixed with the remainder of the investment composition to be the most practical method of applying the invention,

I give' time of illustration and without inany sense restrictingthe invention thereto.

trample 1 7 following examples, purely by way 65% of fine sand, mixed with 22% of calcined gypsum as a binder, and 13% of a mixture of equal parts of finely powderedm'etallic nickel and finely powdered metallic copper. This particular composition has been found toaiford adequate protection to precious metal tooth backings composed of analloy. of gold, platinum, palladium and silver, for a period of 8 hours, at a temperature much above the melting point of .tin. Alloys containing 'at least two of the above named metals together with common base metals such as copper, nickel, etc., in the form sulphate binders, or added to regulate the set of of insets as above described, are completely protected over all practical ranges of heat treatof a mixture of equal quantities of zinc oxide and:

cuprous oxide. This composition has been found to prevent deterioration of the metal baokings noted above, for periods up to 2 hours.

Example 3 herein to include sand, flint,qu artz, cristobalite,

and equivalent materials, reduced to the desired degree of fineness suitable for investment com position.

The above mentionedforms of silica are dense,

compact or substantially non-porous, as "com-' pared with diatomaceous earth or kieselguhr, which latter material has been proposed in certain prior art in foundry molding, of copper and analogous metals, to impart a high degree of porosity to the mold. I do not use kieselguhr, but preferably use quartz, sand or similar 'materials as the siliceous filler. The dense silica has been exclusively used in the actual practice heretofore, as the refractory oxide component of dental investment compositions.

As stated in the parent application, 642,317 (now Patent 1,953,075) the investment composition should have such thermal expansion properties as to counteract for shrinkage of gold on cooling the casting to room temperature.

As was originally stated in saidparent case,

, While the general proportions ofthe ingredients mainly used have been stated, manufacturers in the past have employed the main materials in varying proportions in attempts to meet the varying expressed requirements of the dental profession, such as the time required for the plastic mass to air set, the expansion produced while air setting, the thermal expansion produced dur ing heat treatment, the compressive strength at service temperature, and the degree -ofcoarseness or fineness of the materials."

With diatomaceous earth or 'kieselguhr used as the principal siliceous content of the investment composition, the desired thermal expansion properties would be lost, due to the porous character of the said diatomaceous earth. Quartz and silica sand are well known to be crystalline presence of silica in the investment composition.

In some cases sulphur derived from the gas used in the gas burner may also cause injury to the metal insets, as also sulphur or perhaps other deleterious substances produced from substances added as a retarder'or accelerator for the calcium lium is poisonous.

the composition as a whole. The preferred metals, to be used, in the to of metals or oxides or easily reducible sulphurfree salts, as protective agents are the following (with atomic weights given approximately) V =5'1, Cr=52,' Mn=55, Fe=56, 00:59, Ni=59, Cu- 63, Zn=65.4, Mo=96, Ag=108, W=184.-

It will be understood that there are many other metals or elements which are capable of combining at some temperatures, with sulphur in a reduced condition, but many of these would be im practical for one reason or another. ples, the following may be mentioned':'

Such metals as lead,tin, cadmium, antimony,

bismuth, caesium, gallium, indium and perhaps others, melt at temperatures commonly used in heating the investment and might injurethe precious metals used in the process. Arsenic and osmium, when heated, may evolve offensive or poisonous vapors or fumes. Uranium, thorium, radium and some others are radio-active. Thal Mercury is poisonous and is a liquid which would be extremely harmful to precious metals. Magnesium-and certain other metals would react with water to be used with the investment composition, and would oxidize objectionably even before adding the water. Aluminum, gold, platinum, boron, osmium and titanium do not readily combine'with freesulphur to form stable sulphides. No claim is made herein to the addition of the metals mentioned in this paragraph or the simple salts or, oxides thereof, to investment compostions of the type set forth, as the protective reagent.

The metals which I use herein are all metals which form stable sulphides. The. sulphides of most of these metals are in fact-so stable that they occur in nature as minerals. The metals, in the metallic condition mostly melt at temperatures above the highest temperatures commonly used in the investment process. (up to about 1500-1600911?) l V The metals whose atomic weights are listed above, furthermore, areall classified as heavy metals. (1. e. they have a specific gravity above 5) and thereby they are distinguished from light metals; such as alkali metals, alkaline earth metals 'aluminum, magnesium, etc. Heavy metal" as usedherein (as in accepted books on chemistry) is to be understood as meaning a specfic gravity above 5.

The present application is in part a continuation of my copending application Ser. No. 642,317, filed November 11, 1932, (now Patent 1,953,075) In said copending patent, I have claimed the application to investmentcompositions of the present type, of oxides of the alkaline chromium, copper, manganese, .iron, cobalt,

nickel, molybdenum, silver and tungsten, and the oxides of all of these metals, have a substantial protective effect, but that some of these are far more effective than others. Metallic zinc as such is fairly effective, zinc alloys and easily reducible sulphur-free zinc'salts also can beused. Zinc oxide is quite satisfactory. Vanadium in the metallic form is quitesatisfactory but 'is highly expensive. Vanadium oxide is less satisfactory than the metal. Mixtures of two or several of these can also be used, as well as alloys of the above metals with each other or with other As exam-' consisting of chromium, manganese, iron, cobalt, nickel, copper, molybdenum, silver and tungsten, either in a metallic condition or as an easily reducible sulphur-free compound.

13. A method of protecting preshaped pieces of precious metals or alloys thereof whenbeing heated for a protracted period in an investment composed of a composition including calcium sulphate which comprises the step of incorporating with the investment composition a protective reagent as defined in claim 11.

14. That step in the casting of metals which comprises forcing said metal in a molten state into a preshaped cavity in a hardened investment composition while said hardened investment composition is at between 600 and 1800 F'., said investment composition being composed of a refractory oxide and calcium sulphate, and a metalliferous substance containing a heavy metal selected from the group consisting of sulphur-binding metals either in a metallic form or in the form of an easily reducible sulphur-free compound, said group of metals being capable of combining with reduced sulphur as liberated from the calcium sulphate between the aforesaid temperatures.

PAUL FRANCIS COLLINS. 

